Electrostatically actuated oscillating structure with oscillation starting phase control, and manufacturing and driving method thereof

ABSTRACT

An electrostatically actuated oscillating structure includes a first stator subregion, a second stator subregion, a first rotor subregion and a second rotor subregion. Torsional elastic elements mounted to the first and second rotor subregions define an axis of rotation. A mobile element is coupled to the torsional elastic elements. The stator subregions are electrostatically coupled to respective regions of actuation on the mobile element. The stator subregions exhibit an element of structural asymmetry such that the electrostatic coupling surface between the first stator subregion and the first actuation region differs from the electrostatic coupling surface between the second stator subregion and the second actuation region.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/401,681 filed Jan. 9, 2017, which is a divisional of U.S. patentapplication Ser. No. 14/564,237 filed Dec. 9, 2014, which claimspriority from Italian Application for Patent No. TO2013A001014 filedDec. 12, 2013, the disclosures of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to an electrostatically actuatedoscillating structure with control of the oscillation starting phase,and to the corresponding manufacturing method and driving method. Inparticular, according to the present invention, control of theoscillation starting phase is obtained by structurally shaping theoscillating structure.

BACKGROUND

Known to the art are micromechanical mirror structures (or reflectors)made, at least in part, of semiconductor materials exploiting MEMS(microelectromechanical systems) technology.

MEMS reflectors are designed to receive an optical beam and to vary thedirection of propagation thereof, in a periodic or quasi-periodic way.For this purpose, MEMS reflectors include mobile elements formed bymirrors, the positions of which in space are electrically controlled byan appropriate oscillation control signals.

In greater detail, in a generic MEMS reflector comprising a respectivemirror, the control of position of the mirror is of particularimportance for enabling scanning of a portion of space with an opticalbeam, which is made to impinge on the mirror. In particular, the controlof position of the mirror is crucial in the case of resonant MEMSreflectors, in which, in use, the mirror is made to oscillate in asubstantially periodic way about a resting position, the period ofoscillation being as close as possible to the resonance frequency of themirror in order to maximize the angular distance covered by the mirrorduring each oscillation, and thus maximize the extent of the portion ofspace scanned.

For example, United States Patent Application Publication No.2011/0109951 (incorporated by reference) describes a circuit for controlof the position of the mirror of a MEMS reflector of a resonant type,said mirror being arranged for rotating, under the action of a motor ofan electrostatic type, about an axis of rotation. In particular, theMEMS reflector according to US2011/0109951 comprises a fixed supportbody, of semiconductor material, and a mirror, which is constrained tothe fixed support body by a first spring and a second spring.

The fixed support body comprises a first stator subregion and a secondstator subregion, which are connected, respectively, to a first statorelectrode and a second stator electrode, and a first rotor subregion anda second rotor subregion, which are connected, respectively, to a firstrotor electrode and a second rotor electrode. The first and secondstator electrodes enable biasing, respectively, of the first and secondstator subregions, whereas the first and second rotor electrodes enablebiasing, respectively, of the first and second rotor subregions.

The mirror is mechanically set between the first and second springs,each of which has a respective end that is fixed with respect to thefixed support body. In particular, the first and second springs arefixed, respectively, with respect to the first and second rotorsubregions. The mirror and the first and second springs thus form aresonant system, which has a respective mechanical resonance frequency.In general, the mechanical resonance frequency varies in time, forexample on account of variations in temperature.

In greater detail, according to the patent application No.US2011/0109951, the voltages of the rotor electrodes and of the statorelectrodes, and consequently the voltages of the stator and rotorsubregions, are set in such a way as to cause oscillation of the mirrorabout the axis of rotation. For this purpose, the first and second rotorelectrodes are set at a biasing voltage (V_(pol)), while the first andsecond stator electrodes receive a same electrical control signal.

To bring about oscillation of the mirror with a mechanical oscillationfrequency that is as close as possible to the mechanical resonancefrequency, it is necessary to know the mechanical resonance frequency,and it is necessary to generate the pulses of the electrical controlsignal with an appropriate frequency and phase, as a function of theposition of the mirror.

In greater detail, on the basis of the teachings contained inUS2011/0109951, it is possible to control the position of the mirrorusing an electronic driving circuit. In particular, the statorelectrodes are considered as being electrically equivalent to, andcoinciding with, a node S, and the first and second rotor electrodes areset at the biasing voltage V_(pol). The mirror is thus electricallyequivalent to a capacitor with variable capacitance C_(m) which has afirst terminal, electrically coinciding with the node S, and a secondterminal, set at the biasing voltage, which is generated by an amplifieraccording to the virtual-ground principle. The capacitance of thecapacitor with variable capacitance C_(m) depends upon the torsion towhich the first and second springs are subjected, and thus is inverselyproportional to the angular distance of the mirror from the restingposition, where by “resting position” is meant the position of themirror to which there corresponds a zero torsion of the first and secondsprings. During driving, the node S receives electrical voltage pulses.Since the electrical pulses are at a high positive voltage having avalue higher than the biasing voltage V_(pol), a torque of anelectrostatic nature is exerted on the mirror; in this way, the mirroris kept in oscillation.

However, application of the biasing voltage V_(pol) to the first andsecond rotor electrodes does not make it possible to drive the mirror inoscillation with a pre-set phase. In practice, the oscillation phase issubstantially random or, at least in part, subject to offsets thatderive from manufacturing steps that may not be determined beforehand.

The knowledge of the oscillation phase is important since theorientation of the mirror defines, in use, the direction of the imagesprojected. Thus, it is necessary to know the orientation (or phase) ofthe mirror to generate correct images. According to an embodiment of aknown type, described in US 2011/0181931, the motion of the mirror isdetected by a microphone configured to transduce into an electricalsignal the pressure waves generated by oscillation of the mirror. Inthis way, according US 2011/0181931, it is possible to recognize inwhich position the mirror is in use, and, consequently, derive theoscillation phase thereof.

Even though this system and method provide reliable results, theyrequire integration of a MEMS microphone in the same package as the onethat houses the mirror, with consequent increase in costs and difficultyin containment of the dimension of the final package.

Further, the oscillation phase is not known immediately, but only afteroscillation of the mirror has started, at the end of the startingtransient.

There is accordingly a need to provide an electrostatically actuatedoscillating structure with control of the absolute oscillation startingphase, and corresponding manufacturing method and driving method, thatwill overcome at least in part the drawbacks of the known art, and inparticular will make it possible to govern start of oscillation of theoscillating structure of the MEMS device with a pre-set and known phase.

SUMMARY

In accordance with an embodiment, an electrostatically actuatedoscillating structure and corresponding manufacturing method and drivingmethod are provided.

In an embodiment, an electrostatically actuated oscillating structurecomprises: a fixed support body forming a first stator subregion, asecond stator subregion, a first rotor subregion and a second rotorsubregion; a first torsional elastic element and a second torsionalelastic element mechanically coupled, respectively, to the first andsecond rotor subregions and defining an axis of rotation parallel to afirst direction of an orthogonal reference system; and a mobile elementarranged between, and connected to, said first and second torsionalelastic elements, the mobile element being rotatable about the axis ofrotation as a result of a torsion of the first and second torsionalelastic elements. The first and second stator subregions areelectrostatically coupled to respective first and second regions ofactuation of the mobile element, which are diametrically opposite to oneanother with respect to the axis of rotation. One among the first andsecond stator subregions includes at least an element of structuralasymmetry such that the surface available for said electrostaticcoupling between the first stator subregion and the first actuationregion is greater than the surface available for the electrostaticcoupling between the second stator subregion and the second actuationregion.

In an embodiment, a MEMS projective system comprises: an oscillatingstructure of the type described above and a reflecting element,mechanically coupled to the mobile element, designed to reflect a lightbeam.

In an embodiment, a method for manufacturing an electrostaticallyactuated oscillating structure comprises: (a) shaping a fixed supportbody to form a first stator subregion and a second stator subregion anda first rotor subregion and a second rotor subregion; (b) forming afirst torsional elastic element and a second torsional elastic element,constrained, respectively, to the first and second rotor subregions anddefining an axis of rotation; (c) forming a mobile element set between,and connected to, said first and second torsional elastic elements, themobile element being rotatable about the axis of rotation as a result ofa torsion of the first and second deformable elements; and (d) formingfirst and second regions of actuation of the mobile element, which arediametrically opposite to one another with respect to the axis ofrotation. The steps (c) and (d) include electrostatically coupling thefirst and second stator subregions to the first actuation region and tothe second actuation region, respectively. Further steps include: (e)forming, in an area corresponding to one between the first and secondstator subregions, an element of structural asymmetry such that thesurface available for said electrostatic coupling between the firststator subregion and the first actuation region is greater than thesurface available for the electrostatic coupling between the secondstator subregion and the second actuation region.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, preferredembodiments are now described, purely by way of non-limiting example,with reference to the attached drawings, wherein:

FIG. 1 shows a block diagram of an electronic driving circuit of a MEMSreflector, according to the known art;

FIG. 2 shows a top plan view of a MEMS reflector according to oneembodiment;

FIG. 3 shows a perspective view of the MEMS reflector of FIG. 2;

FIG. 4 shows a perspective view of a MEMS reflector according to afurther embodiment;

FIG. 5 shows a cross-sectional view of the MEMS reflector of FIG. 2,FIG. 3, or FIG. 4;

FIG. 6 shows a perspective view of a MEMS reflector according to afurther embodiment; and

FIG. 7 shows a cross-sectional view of the MEMS reflector of FIG. 6.

DETAILED DESCRIPTION OF THE DRAWINGS

According to an aspect, an electrostatically actuated oscillatingstructure (or a MEMS device including an oscillating mobile element) isprovided, which comprises: a fixed support body forming a first statorsubregion and a second stator subregion and a first rotor subregion anda second rotor subregion; a first torsional elastic element and a secondtorsional elastic element (or springs) constrained, respectively, to thefirst and second rotor subregions and defining an axis of rotationparallel to a direction x of an orthogonal reference system; and amobile element set between, and connected to, said first and secondtorsional elastic elements, the mobile element being rotatable about theaxis of rotation as a result of a torsion of the first and seconddeformable elements. The first and second stator subregions areelectrostatically coupled to respective first and second regions ofactuation of the mobile element (for example, by fingers arranged incomb-fingered fashion). The first and second mobile-element actuationregions are diametrically opposite to one another with respect to theaxis of rotation.

According to the present invention, the first and second statorsubregions are structurally asymmetrical with respect to one another.For this purpose, one between the first and second stator subregionsincludes at least one element of structural asymmetry such that thesurface available for electrostatic coupling between the first statorsubregion and the first actuation region is greater than the surfaceavailable for electrostatic coupling between the second stator subregionand the second actuation region.

It is evident that the element of structural asymmetry has at least onedimension greater than any possible process spread or error to which theoscillating structure may be subject on account of the manufacturingprocess used (depending upon the machinery used, thelithographic/etching processes chosen, etc.).

In particular, the element of structural asymmetry extends in an area ofa portion of the first stator subregion or of the second statorsubregion directly facing a respective portion of the first or secondmobile-element actuation region. In this way, the electrostatic forcesthat are exerted between the first stator subregion and the firstmobile-element actuation region, and between the second stator subregionand the second mobile-element actuation region, are unbalanced withrespect to one another. This causes a net force component that may bedetermined beforehand and is such that the mobile element (given apredefined oscillation control signal) will always start to oscillate inthe same direction or, in other words, with the same phase.

FIG. 1 shows a MEMS projective system 20, of a type in a per se knownmanner, which includes a light source 22 formed, for example, by aplurality of LEDs 24, each of which emits electromagnetic radiation at acorresponding wavelength. For example, FIG. 1 shows three LEDs 24, eachof which emits radiation, respectively, around the red (620-750 nm), thegreen (495-570 nm), and the blue (450-475 nm).

The MEMS projective system 20 further comprises a combiner 26, a MEMSdevice 30, and a screen 32. The combiner 26 is set downstream of thelight source 22 for receiving the electromagnetic radiation emitted bythe LEDs 24 and form a single optical beam OB1, obtained by combinationof said electromagnetic radiation. The combiner 26 is further designedto orient the optical beam OB1 onto the MEMS device 30. In turn, theMEMS device 30, described in greater detail hereinafter, is designed togenerate a reflected optical beam OB2 and to send the reflected opticalbeam OB2 onto the screen 32 for enabling formation of images on thescreen 32.

In detail, the MEMS device 30 is designed to vary in time theorientation in space of the axis of the reflected optical beam OB2 forscanning periodically portions of the screen 32. In particular, thereflected optical beam OB2 linearly scans a portion of the screen 32,possibly the entire screen.

FIGS. 2 and 3 show an embodiment of a MEMS device 30 designed for beingused in the MEMS projective system 20 of FIG. 1.

As shown in FIGS. 2 and 3, the MEMS device 30 comprises a fixed supportbody 40, in particular of semiconductor material, which includes a firststructural region 42 and a second structural region 43, which areelectrically insulated from one another.

The MEMS device 30 is shown in an orthogonal reference system formed bythree axes x, y, z. It is further defined an axis H parallel to the axisz of the orthogonal reference system. The first structural region 42comprises a first rotor subregion and a second rotor subregion 44, 46,which are arranged diametrically opposite with respect to the axis H,and aligned to one another along an axis O parallel to the axis x.Further, the second structural region 43 comprises a first statorsubregion 48 and a second stator subregion 50, which are also arrangeddiametrically opposite with respect to the axis H, and aligned to oneanother in a direction parallel to the axis y. The first statorsubregion 48 is delimited at the top by a front surface 48 a and at thebottom by a rear surface 48 b. Likewise, the second stator subregion 50is delimited at the top by a front surface 50 a and at the bottom by arear surface 50 b.

In the embodiment shown in FIG. 2, the first stator subregion 48includes a massive portion 49 a and a plurality of elongated elements 49b that protrude (or, in other words, extend in cantilever fashion) fromthe massive portion 49 a and have a main direction of extension alongthe axis y. In what follows, said elongated elements 49 b are referredto as “fingers” 49 b.

The second stator subregion 50 includes, in a way similar to the firststator subregion 48, a massive portion 51 a and a plurality ofrespective elongated elements 51 b, which protrude (or, in other words,extend in cantilever fashion) from the massive portion 51 a and have amain direction of extension along the axis y. In what follows, theelongated elements 51 b are referred to as “fingers” 51 b. The fingers49 b extend parallel to the axis y starting from respective regions ofthe massive portion 49 a of the first stator subregion 48 in thedirection of the second stator subregion 50, and are aligned to oneanother in a direction parallel to the axis x. Likewise, the fingers 51b extend parallel to the axis y starting from the massive portion 51 aof the second stator subregion 50, in the direction of the first statorsubregion 48, and are also aligned to one another in a directionparallel to the axis x.

The fixed support body 40 defines a cavity 52. In addition, the MEMSdevice 30 comprises a mobile body 54 that is constrained to the firstand second rotor subregions 44, 46 and is further suspended over thecavity 52.

The mobile body 54 is mechanically and electrically coupled to the firstand second rotor subregions 44, 46 by a first deformable element 56 anda second deformable element 58, respectively. The mobile body 54 has amobile element 60 (that forms a central portion of the mobile body 54),for example circular in top plan view (in the horizontal plane xy), onwhich a mirror layer 65 is arranged, constituted by a material with ahigh reflectivity for the light radiation for being projected, such asfor example aluminum or gold.

The mobile element 60 is connected between the first and seconddeformable elements 56, 58. It should be noted that, typically, themobile element 60, the first and second deformable elements 56, 58 andthe first and second rotor subregions 44, 46 form a single piece, i.e.,belong to a monolithic structure, in particular of semiconductormaterial obtained by known micromachining techniques.

In greater detail, a first end of the first deformable element 56 isfixed with respect to the first rotor subregion 44, whereas a first endof the second deformable element 58 is fixed with respect to the secondrotor subregion 46. Further, a first end and a second end of the mobileelement 60 are fixed with respect to a second end of the firstdeformable element 56 and to a second end of the second deformableelement 58, respectively.

According to one embodiment, in resting conditions, each between thefirst and second deformable elements 56, 58 has the shape of aparallelepiped, the dimension of which parallel to the axis x is greaterthan the corresponding dimensions along the axes y and z; for example,the dimension parallel to the axis x is at least five times greater thanthe dimensions along the axes y and z. In resting conditions, eachbetween the first and second deformable elements 56, 58 has two faces.

For practical purposes, the first and second deformable elements 56, 58function, respectively, as first spring and second spring, since each ofthem may undergo a torsion about the axis O, and then return into theposition assumed in resting conditions.

During torsion of the first and second deformable elements 56, 58, thetwo faces thereof that, in resting conditions, are arranged in planesparallel to the plane xy, are moved from the resting position, rotatingabout the axis O. In fact, the shape of the first and second deformableelements 56, 58 bestows upon them a low torsional stiffness, forexample, comprised between 10⁻⁴ and 10⁻³ N/rad. The mobile element 60and the mirror 65 are thus designed to rotate, in use, about the axis O.

In the embodiment shown in FIG. 2, the mobile element 60 defines arespective plurality of elongated elements (fingers) 61, 63, whichextend parallel to the axis y and are arranged in such a way that, inresting conditions, they are comb-fingered with the elongated elements(fingers) 49 b and, respectively, 51 b, carried by the first statorsubregion 48 and, respectively, by the second stator subregion 50.

The MEMS device 30 further comprises a first stator electrode 62 and asecond stator electrode 64, which are set in contact with the first andsecond stator subregions 48, 50 and enable electrical biasing of thelatter. In addition, the MEMS device 30 comprises a first rotorelectrode 66 and a second rotor electrode 68, which are set in contactwith the first and second stator subregions 44, 46 respectively, andenable biasing of the latter.

One between the first and second stator subregions 48, 50 isstructurally asymmetrical with respect to the other between the firstand second stator subregions 48, 50.

In particular, according to the embodiment shown in FIGS. 2 and 3, thesecond stator subregion 50 presents a reduction of thickness startingfrom the front surface 50 a in an area of the portion of the massiveregion 51 a that extends in the proximity of the fingers 51 b, and alsoin an area corresponding to the fingers 51 b themselves. In other words,following the front surface 50 a along the axis y towards the fingers 51b, a step is present such that the thickness of the massive region 51 adecreases in the area of said step.

In other words, according to this embodiment, the second statorsubregion 50 comprises one or more recesses (designated in FIGS. 2 and 3by the reference number 70) designed to render the first and secondstator subregions 48, 50 structurally asymmetrical with respect to oneanother, in particular when observed in side view along a cross-sectionobtained by sectioning the MEMS device 30 parallel to the plane yz. Asingle recess 70 may extend throughout the length of the second statorsubregion 50 along the axis x (FIG. 4), or else a plurality of recesses70 (in particular two recesses) may be formed each in an areacorresponding to the portions of the second stator subregion 50 fromwhich the fingers 51 b depart, as shown in FIGS. 2 and 3.

Alternatively, it is further possible to form a recess (not shown) onlyin the area of a portion of the second stator subregion 50 extending(along the axis x) between the portions of the second stator subregion50 itself from which the fingers 51 b depart, but not in the area of thefingers 51 b or of the portions of the second stator subregion 50 fromwhich the fingers 51 b depart. In this case, said recess may also extendin the second stator subregion 50 for a depth much greater than what hasbeen described with reference to the recess 70.

It is also possible to remove completely along z a portion of the secondstator subregion 50 extending (along x) between the fingers 51 b.

It is evident that the recess 70 has been previously described, andshown in the figures, as extending in the area of the front surface 50 aof the second stator subregion 50 and in the area of the surface of thefingers 51 b that extends as a prolongation of the front surface 50 a.However, it is evident that the reduction in thickness of the fingers 51b may be obtained also by machining said fingers 51 b at a surfacethereof diametrically opposite (i.e., opposite along z) to the surfaceof the fingers 51 b, which extends as prolongation of the front surface50 a.

Further, it is evident that the recess 70, here shown as being formed inthe area of the second stator subregion 50, may alternatively be formedin the area of the first stator subregion 48, indifferently.

In the case of FIG. 3, each recess 70 is laterally delimited by wallsthat surround it on three sides. The fourth side, free from walls,extends in an area corresponding to the regions from which the fingers51 b depart. In any case, the reduction of thickness that may be notedat the recess 70 is present also at the fingers 51 b.

Irrespective of whether the embodiment of FIG. 3 or that of FIG. 4 isconsidered, the first and second stator subregions 48, 50 have the samethickness when measured in the direction z and, as regards the secondstator subregion 50, outside the recess 70. For example, said thicknessis comprised approximately between 40 μm and 65 μm. The depth of therecess 70, measured in the direction z starting from the front surface50 a, is chosen of a value such as for being greater than the possibleprocess spreads. For example, the depth of the recess 70 is comprisedapproximately between 0.8 μm and 2 μm, in particular 1 μm. In otherwords, generalizing, the depth of the recess 70 is comprisedapproximately between 1% and 5% of the thickness of the first and secondstator subregions 50. As has already been said, said reduction ofthickness applies also to the fingers 51 b, which have a maximumthickness equal to, or smaller than, the maximum thickness, measured atthe recess 70, of the second stator subregion 50.

According to alternative embodiments (not shown), the reduction ofthickness (recess 70) extends only in the area of the fingers 51 b orelse only in the area of the massive portion 51 a.

Driving of the MEMS device 30 takes place in a way in a per se knownmanner. For example, the MEMS device 30 may be driven by excitation ofthe rotational and out-of-plane modes, by biasing the first and secondstator regions 48, 50 at the same voltage (e.g., ground referencevoltage) and the first and second rotor regions 44, 46 at a voltagedifferent from the biasing voltage of the first and second statorregions 48, 50 (e.g., a voltage approximately 20 V higher, in modulus,than the ground reference voltage). To optimize the actuationefficiency, the driving frequency is chosen equal to the resonancefrequency of the mobile mass.

According to a different driving method, the MEMS device 30 may bedriven by excitation of the in-plane mode by biasing the first (orsecond) stator subregion 48 (50) and the first and second rotor regions44, 46 at one voltage (e.g., ground reference voltage), and the second(or first) stator subregion 50 (48) at a voltage different from thebiasing voltage of the first and second rotor regions 44, 46 (e.g., avoltage approximately 20 V greater, in modulus, than the groundreference voltage).

It has been verified that a MEMS device 30 actuated electrostatically bya comb-fingered structure, in particular of the type shown in FIGS. 2-4,which has a structural asymmetry (in particular in terms of differentthickness) between the two stator subregions 48, 50 that governactuation of the mobile body 54 of the MEMS device 30, shows a net forcecomponent that is directed along the axis z and that acts in the regionof the fingers 61 directly facing the first stator subregion 48 (i.e.,the stator subregion that does not present the recess 70) andcomb-fingered to the fingers 49 b. Thus, during electrostatic actuationof the mobile body 54, the latter is preferably subjected to a forcealong z that acts in the region of the fingers 61 facing the firststator subregion 48, such that the mobile body 54 starts to oscillatealways with the same phase; see, for example, FIG. 5, which is a graphicillustration of what has been described here. FIG. 5 is across-sectional view of the MEMS device 30 taken along the line ofsection V-V of FIG. 2. The dashed arrows represent the forces ofelectrostatic coupling (attraction/repulsion) between fingers 61, 63comb-fingered to the fingers 49 b, 51 b, whereas the solid arrowrepresents the force vector F_(p) that acts on the mobile body 54 in thearea of the fingers 61.

This is due to the greater area available for the electrostatic couplingbetween the mobile body 54 and the first stator subregion 48 as comparedto the area available for the electrostatic coupling between the mobilebody 54 and the second stator subregion 50. Consequently, the forces ofattraction/repulsion (according to the polarity of the driving signalsused) between the mobile body 54 and the first stator subregion 48 arehigher, in modulus, than the forces of attraction/repulsion between themobile body 54 and the second stator subregion 50.

It is evident that said effect of control of the oscillation startingphase of the mobile body 54 may be obtained in a way different from whatis illustrated in FIGS. 2-4. In particular, according to an embodimentof the present invention shown in FIG. 6, as an alternative to therecesses 70 of FIG. 3, raised portions are formed, or elements ofincreased thickness, designated by the reference 80, having, in top planview, the shape and spatial arrangement similar to that of the recesses70 (i.e., the top plan view corresponds to that of FIG. 2). The raisedportions 80 are designed to increase the thickness, along z, of part ofthe second stator subregion 50 and extend in areas corresponding toportions of the second stator subregion 50 close to the fingers 51 b;further, also the fingers 51 b have an increased thickness as comparedto the fingers 49 b. In particular, the fingers 51 b have a thicknessgreater than the fingers 49 b by a value equal to the thickness of theraised portion 80 to which they are mechanically coupled; see also FIG.7, which shows a cross-sectional view of the MEMS device 30 of FIG. 6,obtained by sectioning the MEMS device 30 of FIG. 6 with the cuttingplane 82, parallel to yz.

In particular, according to the embodiment shown, the raised portions 80extend in areas of the second stator subregion 50 that correspond, intop plan view, to the portions of the second stator subregion 50 inwhich the recesses 70 shown in FIG. 3 extend.

It is evident that the element of increased thickness 80, hererepresented as being formed in the area of the second stator subregion50, may alternatively be formed in the area of the first statorsubregion 48.

According to a further aspect (not shown) it is possible to envisageformation of a recess 70 in the area of the first stator subregion 48and simultaneously a raised portion 80 in the area of the second statorsubregion 50, or vice versa.

Irrespective of the embodiment, the first and second stator subregions48, 50 have the same thickness when measured in the direction z and, asregards the second stator subregion 50, outside the raised portion 80.For example, said thickness is comprised approximately between 40 μm and65 μm. The thickness of the raised portion 80, measured in the directionz starting from the front surface 50 a, is chosen of a value such as forbeing higher than the possible process spreads. For example, thethickness of the raised portion 80 is comprised between 0.8 μm and inparticular 1 μm. In other words, generalizing, the thickness of theraised portion 80 is comprised between 1% and 5% of the maximumthickness reached by the first and second stator subregions 50(obviously, on the outside of the raised portion 80).

In use, in the case of FIGS. 6 and 7, the second stator subregion 50 andthe fingers 51 b present a surface for electrostatic coupling with thefingers 61 of the mobile body 53 that is greater as compared to thesurface for electrostatic coupling of the first stator subregion 48 andfingers 49 b. Consequently, in this case, the component of force F_(p)directed along the axis z acts in the area of the fingers 61, i.e., ofthe portion of the mobile body 53 directly facing the second statorsubregion 50. Also in this case, as has been described previously, theoscillation starting phase of the mobile body 53 may be determinedbeforehand.

The MEMS device 30 further comprises an electronic control circuit 70(not shown), which is designed to start, and then maintain, oscillationof the mobile body 53.

The electronic control circuit 70 is configured to generate, in a per seknown manner, a driving signal V_(p), formed by a pulse train, which isapplied to the stator electrodes 62, 64. Each pulse has a positivevoltage in the region of, for example, 150-200 V, in such a way that,when a pulse is applied to the first and second stator electrodes 62,64, the mobile element 60 is at a voltage markedly lower than thevoltage of the first and second stator subregions 48, 50, and inparticular at least 20 V less, preferably 50 V less. An electrostaticforce is thus generated that tends to attract towards one another theplates of the variable capacitor C_(var), i.e., the mobile element andthe stator subregions. Further, if we designate by f_(o) the mechanicaloscillation frequency of the mobile element 60, it may be found that thedriving signal V_(p) has an electrical frequency equal to 2·f_(o).

In detail, the driving signal V_(p) has a duty cycle of 50% and issupplied to the first and second stator electrodes 62, 64.

Application of each electrical pulse to the first and second statorelectrodes 62, 64 causes application of a corresponding torque of anelectrostatic nature, which keeps the mobile element 60 in oscillation.In fact, assuming that the mobile element 60 is oscillating and is at anangular distance +θ_(max), application to the first and second statorelectrodes 62, 64 of a first electrical pulse causes generation of atorque that tends to bring the mobile element 60 back into the restingposition, with consequent reduction of the torsion to which the firstand second deformable elements 56, 58 are subjected. Once the restingposition has been reached, application of the torque ceases, but themobile element 60, on account of own inertia, passes beyond the restingposition, until it reaches an angular distance −θ_(max), at which asecond electrical pulse is applied to the first and second statorelectrodes 62, 64. A further torque is thus generated, which tends tobring back the mobile element 60 towards the resting position, and soon. The electrical pulses are thus distributed in time in such a way asto maintain the mobile element 60 in oscillation about the axis O.

The electronic control circuit 70 further manages start of oscillationof the mobile element 60. For this purpose, the driving signal generatedby the electronic control circuit 70 during the initial start-up stageis a periodic pulsed signal, with an electrical frequency equal to2·f_(RIS)′, where f_(RIS)′ is equal to an expected value of themechanical resonance frequency f_(RIS) of the mobile element 60. Purelyby way of example, the value of f_(RIS)′ may be determined, in a per seknown manner, on the basis of the mechanical characteristics of the MEMSdevice 30. The electronic control circuit 70 is thus able, starting froma state in which the mobile element 60 is stationary, to startoscillation of the mobile element 60; in particular, the oscillation isstarted with a mechanical frequency equal to f_(RIS)′, which is close tothe real value of the mechanical resonance frequency f_(RIS), and with apre-set known phase.

The advantages that the present MEMS device affords emerge clearly fromthe foregoing description. In particular, the MEMS device enablesdetection of the position of the mobile element, and simultaneouslydriving of the mobile element itself, without having to uncouple thestep of detection of the position from the driving step, i.e., from thestep of actuation of the electrostatic motor. In this way, the mirrormay be driven at an electrical frequency equal to twice the mechanicalresonance frequency, in which case it is possible to reduce the voltageof the electrical pulses applied to the stator electrodes, given thesame maximum range of the angular distance θ. Consequently, theelectronic control circuit may be obtained by using a singletechnological process. Further, driving of the mirror may be madewithout any need to resort to a complex processing unit or to AD/DAconverters.

Finally, it is clear that modifications and variations may be made towhat has been described and illustrated herein, without therebydeparting from the scope may of the present invention, as defined in theannexed claims.

In particular, the element of structural asymmetry may consist of, orcomprise, a reduction in length along y of the fingers 51 b with respectto the length along y of the fingers 49 b (or vice versa).

Further, the embodiments have been described, purely by way of example,with reference to the case of a MEMS device designed to reflect anoptical beam in an electronically controllable way, and thus includingat least one mirror. Other uses of the MEMS device described hereinafterare, however, possible. In general, the present invention may apply toany oscillating system in which it is important to know the oscillationphase, in particular MEMS resonant systems.

Further, the fingers may be absent. In other words, it is possible forone or more of the mobile element 60 and the first and second statorsubregions 48, 50 not to form any elongated element. In this case, therecess 70 (or the element of increased thickness 80) is formed just inthe area of the massive portion 51 a (or 49 a).

It is further possible for the projective system 20 to include, inaddition to the MEMS device 30, an additional MEMS device, set betweenthe MEMS device 30 and the screen 32 and designed to receive thereflected optical beam OB2, to generate a further reflected opticalbeam, and to send this further reflected optical beam onto the screen32. In this way, it is possible to scan the screen 32 with said furtherreflected optical beam, in a first direction and a second direction, forexample orthogonal to one another.

It is further possible for the MEMS device 30 to include further amovement device, for example of an electromagnetic type, designed torotate the mobile element 60, and thus the mirror 65, about a furtheraxis parallel, for example, to the axis y.

Finally, it is possible for at least one between the first and seconddeformable elements for being of a material different from asemiconductor, such as for example a metal.

What is claimed is:
 1. A MEMS projective system, comprising: anoscillating structure; and a reflecting element, mechanically coupled toa mobile element of the oscillating structure, configured to reflect alight beam; wherein the oscillating structure comprises: a first statorsubregion, a second stator subregion, a first rotor subregion, a secondrotor subregion, wherein the first and second rotor subregions arecoupled by torsional elastic elements to the mobile element rotatableabout an axis of rotation; wherein the first and second statorsubregions are electrostatically coupled to respective first and secondrotor subregions, wherein the first and second rotor subregions includea respective plurality of first and second elongated elements, whichextend in cantilever fashion towards the first and second statorsubregions, respectively, the first and second stator subregionsincluding a respective plurality of third and fourth elongated elementscomb-fingered to the first and second elongated elements, respectively,and wherein the first stator subregion includes at least an element ofstructural asymmetry in the form of a protuberance of increasedthickness extending in a direction perpendicular to an upper surface ofthe first and second stator subregions, said protuberance of increasedthickness providing the third elongated elements with a thickness thatis greater than a thickness of the first elongated elements at alocation where the first and third elongated elements are comb-fingered,such that a surface available for electrostatic coupling between thethird elongated elements and the first elongated elements is greaterthan the surface available for the electrostatic coupling between thefourth elongated elements and the second elongated elements.
 2. Thesystem according to claim 1, further comprising: a light source actuatedfor generating a beam of light incident on said reflecting element; andan image-generation module, operatively coupled to said oscillatingstructure, for generating part of an image associated to the light beamreflected by said reflecting element.
 3. The system according to claim1, wherein the fourth elongated elements have a thickness that is equalto a thickness of the second elongated elements at a location where thesecond and fourth elongated elements are comb-fingered.
 4. An apparatus,comprising: a mobile element supported on opposed first and second endsfor oscillation about an axis of rotation, wherein the mobile elementfurther includes: a first region of electrostatic coupling including aplurality of first elongated elements disposed on a first side of themobile element; a second region of electrostatic coupling including aplurality of second elongated elements disposed on a second side of themobile element diametrically opposite the first region across said axisof rotation; a stator body including: a third region of electrostaticcoupling including a plurality of third elongated elements, the firstelongated elements comb-fingered to the third elongated elements; and afourth region of electrostatic coupling including a plurality of fourthelongated elements, the second elongated elements comb-fingered with thefourth elongated elements; wherein the third region is structurallyasymmetric relative to the fourth region because the third regionincludes a protuberance of increased thickness extending in a directionperpendicular to an upper surface of the stator body, said protuberanceof increased thickness providing the plurality of third elongatedelements with a thickness that is greater than a thickness of the firstelongated elements at a location where the first elongated elements arecomb-fingered to the third elongated elements; wherein a first area ofelectrostatic coupling is formed at the location where the firstelongated elements are comb-fingered to the third elongated elements;and wherein a second area of electrostatic coupling is formed at alocation where the second elongated elements are comb-fingered to thefourth elongated elements, the first area being greater than the secondarea because of the protuberance.
 5. The apparatus according to claim 4,wherein the fourth elongated elements have a thickness that is equal toa thickness of the second elongated elements at the location where thesecond and fourth elongated elements are comb-fingered.